Dictionary.com - Firefox Addon Dictionary.com - Firefox Addon - This addon searches for words using Dictionary.com, a free online service. Dictionary.com will give definitions from many different sources, including Merriam-Webster and American Heritage Dictionary, along with Dictionary.com Unabridged
1TuDien - Firefox Addon 1TuDien was born with a desire Dictionary online tool ready for everyone to work or study. There are five languages for Dictionary and Interface, including: Vietnamese, English, French, Japanese, Chinese; and specialized dictionaries, directories are built and added daily. 1TuDien also supports translating with 1640 pairs of languages
DiiCT (Multilingual Dictionary) - Firefox Addon DiiCT (Multilingual Dictionary) - Firefox Addon - This addon searches for translations using DiiCT.com, a free online multilingual dictionary. DiiCT.com currently supports translation between the following languages: English, German, French, Italian, Spanish, Dutch, Portuguese, Czech, Polish, Russian and Armenian.
The dictionary DiiCT
OneLook (all dictionaries) - Firefox Addon OneLook (all dictionaries) - Firefox Addon - The OneLook website indexes word definitions from a whole host of online dictionaries, meaning that if one dictionary doesn't have the definition you are looking for, OneLook's search might well find the word definition in another dictionary, too